A leading process for making high-quality glass substrates with pristine surface quality for LCD displays is the fusion down-draw process. The forming device in a fusion down-draw process, typically called an isopipe, is illustrated in FIG. 1. The isopipe 100 illustrated in this Figure comprises a trough 103 and a wedge 107. Glass melt is introduced into the trough through an inlet tube 101. A stream of glass melt is allowed to flow over each side of the trough, down on both sides of the trough and wedge. The two glass streams join at the bottom of the wedge 109, where they fuse together to form a single glass sheet 111 having two pristine surfaces that have not been exposed to the isopipe surface during forming. In a typical method to make the isopipe, zircon is isostatically pressed in a machine called isopress into a larger block (called “green body”) and then fired at a high temperature, such as over 1500° C. During firing, zircon crystal grains grow and pack, transforming the green body into a dense ceramic body. Significant shrinkage of the green body is typically observed during firing. The fired, dense ceramic block having a relatively stable structure and density under normal operating conditions of the isopipe is then cut into the shape and size for an isopipe.
The size of LCD glass substrates used by LCD panel makers has increased steadily over the years. The wider the glass substrate, the wider the isopipe is required. Traditionally, an isopipe is machined from a single, unitary piece of zircon block. Understandably, larger isostatic presses are required to make wider zircon block suitable for larger size isopipes based on a unitary piece of zircon ceramic material. With the machining of the fired zircon block and the shrinkage of the green body during firing taken into consideration, the isopress required can be significantly larger as the isopipe size increases from one generation to the next. The high capital investment for larger isostatic presses could be prohibitive, especially for large size glass substrates, such as those of Gen-10 (2850×3050 mm), Gen-11, G-12 or above. In addition, processing much larger and heavier blocks could pose difficult technical problems, such as a higher potential for crack formation, non-uniformity of grain distribution, lower green and fired density, and lower strength, among other issues that can become more protracted with increasing article size.
Hence there is a need for an effective and efficient process for making large-size zircon ceramic bodies. The present invention satisfies this and other needs.